1. Field of the Invention
The present invention relates to a method for forming a plating film and to an electronic component having a plating film formed thereon by the same method. More particularly, the present invention relates to a method for forming a plating film which is lead-free and excellent in solderability and to an electronic component having a plating film formed thereon by the same method.
2. Description of Related Art
In the field of the electric and electronic industries, where soldering is generally used for formation of circuits, an Sn—Pb, containing alloy is typically used as a soldering material and for rapid and ensured soldering, a plating film of the Sn—Pb-containing alloy is applied onto an object to be soldered before hand.
For example, as shown in FIG. 7, a photo coupled semiconductor device 40, which utilizes soldering, is constituted as follows. A light emitting element 42 and a photoreceptor 43 are connected to headers of lead frames 41, respectively, by die bonding. The lead frames 41 are made of a metal such as a Cu alloy or Fe alloy and are bent in advance. The light emitting element 42 and the photoreceptor 43 are connected to the adjacent lead frames 41, respectively, by wire bonding using gold wires 48. The light emitting element 42 is precoated with resin 44. The lead frames 41 having the light emitting element 42 and the photoreceptor 43 connected thereto by die bonding are positioned and opposed to each other by spot welding or using a loading frame set, and an inner package is formed by primary transfer molding using a light transmitting epoxy resin 45. Further, an outer package is formed by secondary transfer molding using a light shielding epoxy resin 46, and a non-glossy (non-shiny or lusterless) SnPb plating film is applied onto portions of the lead frames 41 extending outside the outer package.
Thus, SnPb, which is inexpensive, has a low melting point, and is excellent in processibility, has been used as a material for soldering and also as a material for a plating film. In recent years, however, it becomes urgent to replace SnPb by a material which does not contain lead (lead-free material), as an environmental protection measure. Examples of such a material for soldering include an alloy containing tin as a principal component together with one or more selected from silver, bismuth, copper, zinc and the like, and examples of such a material for a plating film include an alloy containing tin as a principal component together with one or more selected from silver, bismuth, copper, zinc and the like; gold; silver; palladium; and the like.
Japanese Unexamined Patent Publication No. 2000-26994 proposes an SnCu plating coat layer whose cryatal on a surface is fined so that the surface is in a gloss state or a half gloss state.
However, it has been found that a plating coat layer as mentioned above has a zero cross time of 5 seconds or more, indicating that the coat layer is poor in solderability and appearance when the solderability of the plating coat layer is evaluated by a meniscograph method by immersing a component which has the coat layer as mentioned above into a soldering bath filled with a soldering alloy comprising Sn—Ag (3.5%) at 270° C., at a predetermined rate and to a predetermined depth for five minutes and then picking it upward.